System and method for texturing magnetic data storage disks

ABSTRACT

In the texturing unit of this invention a data storage disk is gripped between a pair of opposed, counter-rotating cylindrical abrasive mandrels, which are covered with soft, porous pads. The frictional force between the disk and the pads raises the edge of the disk against a pair of driven rollers which impart a rotational motion to the disk. A texture pattern of uniform grooves, having controlled peak heights and valley depths, is formed simultaneously on both sides of the disk. A system for delivering an abrasive slurry to the pads includes a closed loop in which the slurry is recirculated and a dispense section of that loop, from which a blast of pressurized air periodically expels the slurry through applicators and onto the pads. The continuous recirculation of the slurry prevents settling. 
     A unique feature is that the groove length may be controlled by adjusting the ratio between the rotational speeds of the disk and mandrels. The groove pattern is primarily circular but a radial component may be added by varying the position at which the pads contact the surfaces of the disk.

This application is a division of application Ser. No. 07/842,695, filedFeb. 27, 1992.

FIELD OF THE INVENTION

This invention relates to the media used in disk drives and inparticular to systems and methods of providing a texture on a magneticdata storage disk.

BACKGROUND OF THE INVENTION

Most high capacity disk drives used in computer systems employ thin filmmagnetic media for storing binary encoded data. In a typical disk drive,a magnetic disk is rotated at a high speed about a central axis and thedata are written and read by a magnetic head. In storing and retrievingdata from the magnetic storage disk, the magnetic read/write headtypically rides on a thin cushion of air as it moves among the datatracks along the surface of the rapidly spinning disk.

In many disk drives, the head rests on the surface of the disk when thedrive is turned off. When the disk starts spinning, the head slidesalong its surface for some distance until the disk reaches a rotationalspeed at which the head becomes airborne. The reverse process takesplace when the disk is brought to a stop. This sliding contact betweenthe head and the disk may result in damage to the disk and therefore, toreduce wear and friction, a thin film of lubricant is normally appliedto the disk surface.

Problems of stiction may also occur. If the disk topography is toosmooth, the head will "weld" to the surface while the disk is at rest,and if the lubricant film is too thick, the surface energy of the filmwill "bond" the head to the disk surface.

It has been found that these problems are minimized if a texture of veryfine grooves, separated by ridges, is created by abrasion on the surfaceof the disk. The grooves may act as reservoirs for the lubricant so thatit can be replenished as it is worn off by contact between the head andthe ridges, and they overcome stiction by preventing the head fromcoming into contact with a continuous flat surface of the disk when itis at rest. The texture is normally formed in a hard layer ofnickel-phosphorous material which overlays the relatively soft aluminumsubstrate of the disk, and the texture is perpetuated as additional thinfilm layers, including the magnetic layer, are deposited on the hardlayer.

A large number of texture patterns are possible, ranging from oneextreme, in which all of the grooves are concentric circles about theaxis of the disk, to the other extreme, in which all of the grooves areoriented radially to the axis of the disk. The direction of the groovesis important in achieving optimum performance, since the grooves mayaffect the direction of easy magnetization and the magnitude of thecoercive force.

For certain thin film magnetic alloy compositions, the anisotropy inmagnetic characteristics which is caused by the grooves benefitsperformance, particularly for high density recording. For thesecompositions, circular grooves tend to yield higher coercivities in thecircumferential direction. This is an advantage, as it is in therotational direction of the head where the highest number of fluxchanges per inch are desired. Moreover, the lower coercivity in theradial direction minimizes "off track" noise. On the other hand, purelycircular grooves may increase the extent of wear as the head slides onthe ridges, while bit shift or phase margin defects may result if thegrooves have too large a radial component. For these reasons, thepresent view is that a pattern which includes primarily circular grooveswith a small radial component is desirable.

Whatever pattern of grooves is selected, it is important that thetexture be extremely uniform. Surface asperities will cause glideproblems and may cause failure in start-stop cycle tests.

Several types of texturing systems are known, generally classified as"fixed abrasive" or "slurry abrasive". Fixed abrasive processestypically use abrasives bonded to a mylar tape. The disk to be texturedis clamped at its inside circumference, and rotated. The tape issupported by a cylindrical surface and pressed against the disk. Whilethis system allows both sides of the disk to be textured simultaneously,it is subject to several disadvantages. First, relatively large clampingand rotational forces are applied, normally at the central aperture ofthe disk, because the abrasive tape is acting as a "brake" on thesurface of the disk. These large forces may cause distortion of thedisk, particularly with the thinner substrates now being introduced.Second, many abrasives are not available in tape form, and someabrasives and pad combinations that are available, such as diamond, areprohibitively expensive. Third, problems with texture uniformity,imbedding, asperities, etc. sometimes require the use of two-step tapeprocesses. This increases the cost.

There are two types of abrasive slurry machines: units which employ apad mounted on a rotating quill wheel, and units which are similar tothe tape machine but use a cloth tape. In both types, a free abrasiveslurry is sprayed onto the surface of the disk. These systems allow awider choice of abrasives and tend to offer a less expensive textureprocess. Moreover, a large selection of pad materials is available forthe rotating quill units. On the other hand, the rotating quill unitscannot texture both sides of a disk simultaneously, are difficult toautomate, and require frequent pad replacement. The cloth tape unitsrequire the disk to be firmly clamped, and the selection of cloth"tapes" is extremely limited. In any slurry machine, the abrasiveparticles in the slurry may settle or agglomerate into larger particles,particularly in the slurry supply lines.

SUMMARY OF THE INVENTION

The system of this invention allows both sides of a disk to be texturedat the same time. It provides a uniform circumferential texture patternwith some radial component. It further allows the size of the radialcomponent to be varied. No strong clamping forces are imposed on thedisk, and there are no lines or reservoirs in which the slurry maysettle. Repeated cleanings of the texturing pad after each disk has beenprocessed prolongs the life of the pad and permits it to be used fornumerous texturing cycles.

In a system according to this invention, a disk is sandwiched betweentwo opposing cylindrical mandrels, typically oriented with their axeshorizontal. The mandrels make contact with elongated regions on bothsides of the disk which extend substantially between two points on theouter circumference of the disk. Frequently, the contact regions willinclude a diameter of the disk. The surfaces of the mandrels contain anabrasive. Preferably, the mandrels are covered with soft removable pads,and the abrasive is applied in the form of a slurry near the contactregions between the pads and the disk.

A means is provided for maintaining the disk in a fixed position betweenthe mandrels while imparting a rotational movement to the disk. In apreferred embodiment, this rotational means comprises two drive rollerswhich are positioned above the disk and make contact with the outer edgeof the disk when it is lifted upward.

The two opposing mandrels are rotated in opposite directions so as toexert an upward frictional force in their region of contact with thedisk. This lifts the disk upward until its edge contacts the two driverollers, which impart a rotational motion to the disk. When this hasoccurred, the disk is abraded in a generally upward direction by theabrasive particles on the texturing mandrels as it is being rotated bythe drive rollers. The result is a texture pattern which is largelycircular but contains radial components.

According to another aspect of the invention, when the texturing processis completed, a pair of brushes or other conditioning or cleansing meansscrubs the texturing mandrels as they are moved apart to release thedisk. Water may be sprayed onto the texturing mandrels at the same timeto assist the scrubbing process.

According to another aspect of the invention, an abrasive slurry isapplied to the disk or the texturing mandrels. A slurry reservoir and apump are connected into a closed slurry recirculation loop. A pair ofslurry feed lines are connected to respective points in therecirculation loop, defining a dispense section, and extend to slurryapplicators (e.g., nozzles) which are positioned on either side of thedisk. An air supply line, which runs from a source of pressurized air,is connected to the slurry recirculation loop at a point in the dispensesection.

When slurry is not being applied to the disk or rollers, valves in thefeed lines and the air supply line are closed. When the slurry is to beapplied, these valves are opened, and one or more valves locatedelsewhere in the slurry recirculation loop are closed. As a result, highpressure air flows through the air supply line and enters therecirculation loop, forcing slurry that is located in the dispensesection into the feed lines and through the applicators.

The air continues to flow until all of the slurry has been expelled fromthe feed lines and applicators. When this has happened, the valves inthe air supply line and the feed lines close, and the valves in therecirculation loop open. The pump then recirculates the slurrycontinuously through the recirculation loop again.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in perspective a disk texturing unit in accordancewith the invention.

FIG. 2 illustrates the disk lifter.

FIG. 3A is a perspective view similar to FIG. 1 illustrating the padcleaning brushes.

FIG. 3B is a front elevational view of the structure shown in FIG. 3A.

FIG. 3C is a side elevational view of the structure shown in FIG. 3A.

FIGS. 4A and 4B are front views of the mandrel position controlmechanism in open and closed positions, respectively.

FIGS. 4C and 4D are perspective views of the mandrel position controlmechanism in open and closed positions, respectively.

FIG. 4E illustrates the drive mechanism for the cleaning brushes.

FIGS. 5A, 5B and 5C illustrate the contact region between a texturingmandrel and a disk in three possible configurations, respectively.

FIG. 6A illustrates the variation of pressure across the contact region,and FIG. 6B is an illustrative cross-sectional view of a groove formedin a disk.

FIG. 7 illustrates a texture pattern in accordance with the invention.

FIG. 8 illustrates schematically an abrasive slurry delivery system inaccordance with another aspect of the invention.

DESCRIPTION OF THE INVENTION

A perspective view of a disk texturing unit in accordance with theinvention is illustrated in FIG. 1. In disk texturing unit 10, a disk 11is sandwiched between a pair of cylindrical texturing mandrels 12 whichare driven to rotate in opposite directions about shafts 13 by a drivingmeans (not shown). As shown in FIG. 1, disk 11 has been lifted from acassette 14, containing other disks, by a disk lifter 15. As shown inFIG. 2, a lifter edge holder 16 is mounted on disk lifter 15, and lifteredge holder 16 engages a circumferential edge of disk 11 thereby liftingdisk 11 into the position shown in FIG. 1. A V-shaped groove 16A in edgeholder 16 grips the edge of disk 11 and holds it upright. Disk lifter 15is actuated by a drive means which is not illustrated in FIGS. 1 and 2.

FIG. 1 shows disk 11 after it has been lifted to its upper limit, wherea circumferential edge of disk 11 makes contact with a pair of driverollers 17. Also shown in FIG. 1 are a pair of nozzles 18, which directa stream or spray of a water-based abrasive slurry into the region ofdisk 11 and mandrels 12, a pair of nozzles 18A for spraying a lubricant,and a pair of nozzles 19 for spraying additional solutions to rinse disk11. A nozzle 20 sprays a solution to keep the disks in cassette 14 wet.Mandrels 12 are covered with soft, porous pads 21, which are theelements which actually make contact with the surfaces of disk 11. Theslurry is preferably sprayed from nozzles 18 onto disk 11 but mayalternatively be sprayed onto pads 21 or both disk 11 and pads 21.

Several additional components of texturing unit 10 are illustrated inFIGS. 3A and 3B, which are perspective and front elevational views,respectively, of texturing unit 10. A pair of mandrel position controlmechanisms 22 include bearings 96 and 97 into which the ends of shafts13 are journaled. Mechanisms 22 control the positions of mandrels 12with respect to disk 11, alternately separating mandrels 12 from disk 11or urging mandrels 12 towards each other so that they exert equalopposing forces against the sides of disk 11. In their closed position,mechanisms 22 also control the pressure applied by mandrels 12 to disk11. A pair of brushes 23 are mounted on shafts 24, the ends of which areinserted into bearing blocks 25 (see FIG. 3B). Brushes 23 are positionedsuch that they engage pads 21 when mandrels 12 are separated bymechanism 22.

Texturing unit 10 operates in the following sequence. Initially, disk 11is in cassette 14, and mandrels 12 are held motionless in a separatedcondition by mechanism 22. The texturing process starts when disk lifter15 engages disk 11 and lifts it out of cassette 14 to a position betweenmandrels 12. When disk 11 has been lifted to the point where its centeris substantially between mandrels 12, mechanism 22 causes mandrels 12 toclose against disk 11 and also ensures that mandrels 12 apply equalpressures of a predetermined magnitude against the surfaces of disk 11.Disk 11 is contacted by pads 21. The drive means (not shown) then beginsto rotate mandrels 12 in opposite directions so that a frictional forcebetween pads 21 and the surfaces of disk 11 urges disk 11 upward. Atabout the time mandrels 12 began to rotate, a lubricant is sprayedthrough nozzles 18A onto pads..21 to assist in maintaining a constantlubricant/water ratio on pads 21 following their cleansing (describedbelow).

An upper edge of disk 11 contacts rollers 17, which are driven to rotatein the same direction. Rollers 17 therefore cause disk 11 to rotate(counterclockwise in FIGS. 1-3). It should be noted that the position ofdisk 11 when it contacts rollers 17 is slightly (e.g., 0.25 inches)above its position immediately after the upward stroke of disk lifter 15is completed. Thus, the lower edge of disk 11 is not in contact withdisk lifter 15 as disk 11 is rotated by rollers 17.

It will be evident that a system of more than two rollers (or belts,etc.) could be substituted for rollers 17, and that only one of therollers need be driven.

Simultaneously with the engagement of disk 11 with mandrels 12 androllers 17, an abrasive slurry is sprayed through nozzles 18 onto thesurfaces of disk 11. The slurry clings to (and to some .extent isabsorbed into) pads 21 and causes the surfaces of disk 11 to be abradedas mandrels 12 rotate. At the same time disk 11 is rotated by rollers17, so a texture is formed by the abrasive over the entire surface ofdisk 11.

In the embodiment of FIGS. 1-4, mechanism 22 on the near side (facingthe viewer) closes just before mechanism 22 on the far side. This causesmandrels 12 to grip the right side of disk 11 first, and starts disk 11rotating in a counterclockwise direction. Thus, disk 11 is alreadyrotating to some extent before it engages rollers 17, which accelerateits rotation in that direction. Alternatively, mandrels 12 may betapered slightly (large end facing the viewer) to impart a continuouscounterclockwise motion to disk 11 and thereby assist rollers 17.

Disk 11 is texturized (abraded) for a predetermined period of time, forexample 10 seconds. When the texturizing process is complete, rinsewater is sprayed through nozzles 19 for about 3 seconds. Mandrels 12then stop rotating and mechanisms 22 cause mandrels 12 to separate,thereby releasing disk 11. Disk 11 falls onto disk lifter 15 again,which lowers it into cassette 14.

When mandrels 12 separate, they begin to rotate again and engage brushes23 (see FIG. 3B). Each of brushes 23 is rotated in the same direction asthe mandrel 12 which it contacts. Thus, brushes 23 scrub the remainingabrasive particles from pads 21, and at the same time rinse water issprayed through nozzles 19. The cleansing process normally takes abouttwo seconds and when it is finished mandrels 12 and brushes 23 stoprotating. Mandrels 12 are then ready for the next disk to be textured.

When mandrels 12 engage the next disk, most of the abrasive has beenscrubbed from pads 21 and they are in a relatively clean condition. Thisis important because, as will be described later, uniformity oftexturing requires that a controlled amount of slurry (and abrasive) beused to texture each disk.

To help protect the disks in cassette 14 from slurry which drips fromthe mandrels 12 or disk 11, a cover 30 (shown in FIG. 3C) may beprovided, with a slot 31 to allow movement of disk lifter 15. Inaddition, a solution is sprayed through nozzle 20 to keep the disks incassette 14 wet, and this spraying action also tends to remove anyabrasive particles which might collect on the disks in cassette 14.

FIGS. 4A-4E illustrate the construction of one of mandrel positioncontrol mechanisms 22. Shoulder bolts 103 are fitted through slotsformed in a supporting member 104 and into holes tapped into the bottomof bearings 96 and 97, respectively. Accordingly, bearings 96 and 97 arefree to move towards and away from one another. Bearing 97 is attachedto the end of a shaft 98, which extends to a piston 95A within an aircylinder 95. Shaft 98 has a slot 98A formed in it, through which theright hand shaft 13 extends. Air cylinder 95 is attached to bearing 96.Air cylinder 95 has air inlet ports 110 and 111.

When air is admitted to inlet port 111 (FIG. 4A), piston 95A is drivento the left, and bearing 96 is driven away from bearing 97 untilshoulder bolts 103 reach the outside ends of the slots formed insupporting member 104. When air is admitted through inlet port 110,piston 95A is driven to the right, and bearing 96 and bearing 97 arepulled together until shoulder bolts 103 reach the inside ends of theslots formed in supporting member 104. The inside ends of slots 103 areplaced such that maximum pressure is applied between pads 21 and disk 11when shoulder bolts 103 are in this position. The pressure between pads21 and disk 11 can be reduced by controlling the air flow through port110 so as to prevent shoulder bolts 103 from reaching their innermostpositions.

FIG. 4E illustrates a drive mechanism for rotating the cleaning brushes23 when mandrels 12 are in their open position. Shafts 13 are attachedto respective drive gears 99. A pair of idler gears 101 are mounted suchthat they engage gears 102 which are mounted on shafts 24. Whenmechanism 22 is in its closed position, gears 99 do not engage gears101, and brushes 23 remain motionless. When mechanism 22 is in its openposition (not illustrated in FIG. 4E), gears 99 engage idler gears 101,thereby driving shafts 24 and brushes 23 in a rotational direction thesame as that of mandrels 12. Gears 99, 101 and 102 are sized such thatbrushes 23 make contact with pads 21 when the gears are engaged.

As noted above, pads 21 are soft and porous, and the pressure whichmechanisms 22 cause to be applied between mandrels 12 and disk 11 can beadjusted. As a result, the region of contact between pads 21 and disk 11can be varied, and this in turn alters the texture pattern on disk 11.This is illustrated in FIG. 5A and (in a somewhat exaggerated fashion)FIG. 5B. Regions 50 and 51 represent the region of contact between disk11 and one of mandrels 12 when mechanisms 22 have been adjusted to applydifferent pressures between mandrels 12 and disk 11. It will be apparentthat region 50 represents a smaller pressure than region 51. In a normalsituation the width of the contact region is about 3/16 inch.

Texturing unit 10 is adjusted so that the surfaces of pads 21 are movingconsiderably faster than the surfaces of disk 11. Thus, each abrasiveparticle is trapped between pads 21 and disk 11 at the lower extremityof a contact region and forms a groove as it moves to the upperextremity of the contact region. Referring to FIGS. 5A and 5B, it isapparent that a pattern of grooves in region 1 will depart more from apurely circular orientation than a pattern of grooves in region 50.Thus, the texture pattern formed under the conditions shown in FIG. 5Awill be closely circular, while the texture pattern formed under theconditions shown in FIG. 5B will have a larger radial component. Ingeneral, the radial component will also increase as the rotationalvelocity of mandrels 12 is increased, and will be greater near thecenter of disk 11.

The radial component of the texture pattern may also be enhanced byraising or lowering the position of mandrels 12 with respect to disk 11.FIG. 5C illustrates a contact region 52 which might result from doingthis. From the discussion above, it will be apparent that the texturepattern formed by the configuration illustrated in FIG. 5C will have asignificant radial component.

Several other aspects of the texture pattern formed by unit 10 areworthy of mention. As noted above, pads 21 slide relatively rapidlyagainst the surfaces of disk 11. For example, in one embodiment mandrels12 rotate at 500 rpm while disk 11 rotates at 200 rpm. As this rotationtakes place, the abrasive particles on the surface of pads 21 contactdisk 11 in the contact regions illustrated in FIGS. 5A-5C. In eachinstance, because soft pads 21 are deformed, the pressure between pads21 and disk 11 increases from zero at one edge of a contact region to amaximum at the midpoint of the contact region, and falls to zero at theopposite edge of the contact region. This relationship is illustrated inFIG. 6A. As a result, the grooves formed by the abrasive particles inthe surface of disk 11 have a depth which varies from zero at the endsof the groove to a maximum depth at the midpoint. This structure isillustrated in FIG. 6B, which is an illustrative view not drawn toscale.

It is thus apparent that the texture pattern formed on a disk bytexturing unit 10 comprises a plurality of relatively short grooves (forexample, less than 1.0 inch long) rather than longer grooves whichextend for a significant angular distance around the center of the disk.This eliminates many of the problems which occur when a single abrasiveparticle contacts the disk surface for many revolutions (i.e., particleembedding, redeposition of removed material, asperities and gouges). Thevariation of abrasion pressure from the ends to the midpoint of eachgroove also minimizes many undesirable effects, such as embedding, andtends to give better groove uniformity over the disk surface. Moreover,since the rotational speeds of disk 11 and mandrels 12 are controlledseparately, the length of grooves can be varied. In general, the lengthof the grooves is reduced by increasing the rotational velocity ofmandrels 12 and reducing the rotational velocity of disk 11.

A further advantage is that pads 21 do not contact disk 11 except inrelatively small contact regions such as those illustrated in FIGS.5A-5C. As pads 21 rotate, most of their surfaces are exposed. Thisgreatly extends the life of the pads as compared to the pads used withconventional quill wheel machines. The cleaning mechanism provided bybrushes 23 further extends the life of pads 21.

The width of pads 21 on mandrels 12 may be reduced to any dimension soas to permit a specific region of disk 11, such as a landing zone, to betextured. Two or more texturing units, having pads at differentpositions of their respective mandrels, could be used to providedifferent texture patterns in different annular regions of the disk.This type of patterning is unique and may provide advantages to magneticrecording at higher densities.

No strong clamping forces are imposed on disk 11 by texturing unit 10.The upward frictional force provided by mandrels 12 is just sufficientto ensure firm contact between the edge of disk 11 and rollers 17.Neither this force nor the tangential force applied to the edge of disk11 by rollers 17 is strong enough to cause any significant distortion ofdisk 11.

It is important that a precise measured amount of the abrasive slurry beapplied through nozzles 18 during each texturing cycle, and that theslurry be well mixed. These factors, in conjunction with the cleaning ofmandrels 12 between cycles, assure a uniform texturing of successivedisks.

The slurry preferably contains abrasive particles of alumina, siliconcarbide, diamond, etc. measuring from 0.2 to 20 microns, with 0.5 to 5microns being highly preferred. It also contains water and a cuttinglubricant selected for ease of cleaning and for producing an optimumsurface finish. Water-diluteable formulations without hard-to-removeresidues are desirable. Other ingredients such as surfactants andantimicrobial agents may be added to the slurry.

FIG. 7 illustrates a texture pattern formed on a disk using a disktexturing unit in accordance with the invention. The texture pattern wasmeasured with a Wyco surface profilometer.

The Appendix shows the results of a series of tests that were performedwith the disk texturing unit. The disks used were 130 mm polished nickeldisks, and a diamond slurry was used. The mandrels were 2.20 inches indiameter. The disk edge drive rollers were 1.44 inches in diameter. Thepads with adhesive backing were bonded to the mandrels using pressuresof about 30 psi.

FIG. 8 shows a system for providing a measured amount of a well-mixedslurry during each texturing cycle. Slurry delivery system 80 includes acone-shaped reservoir 81 and a pump 82. Reservoir 81 and pump 82 areconnected into a recirculation loop which includes an outflow line 83, adispense section 84 and a return line 85. Dispense section 84 is definedby junction points 86 and 87 on either end and includes a junction point88 at its midpoint. Slurry feed lines 89' and 90 extend from junctionpoints 86 and 87, respectively, and are connected to slurry nozzles 18.An air supply line 91 extends from a source of pressurized air (notshown) to junction point 88. Each of lines 83 and 85, feed lines 89 and90, and air supply line 91 contains a section of soft plastic or rubbertubing which can readily be pinched so as to close off flow These pinchvalves are labeled 83', 85', 89', 90' and 91' in FIG. 8. Pinch valves83' and 85' are situated above a pinch bar 92, and pinch valves 89', 90'and 91' are situated below pinch bar 92. Hard surfaces are providedadjacent valves 83', 85' and 89'-91' on the sides opposite to pinch bar92. Pinch bar 92 is connected by a connecting rod 93 to an actuator 94,which is capable of lifting and lowering pinch bar 92 so that italternately closes valves 83' and 85', or valves 89', 90' and 91'.

Initially, pinch bar 92 closes valves 89', 90' and 91' and thereforeprevents flow from occurring in slurry feed lines 89 and 90 and airsupply line 91. An abrasive slurry is added to reservoir 81 and iscirculated by pump 82 through outflow line 83, dispense section 84 andreturn line 85. As long as pinch bar 92 remains in this position, theslurry will be continuously circulated, and pump 82 is sized so that theflow characteristics (Reynolds number, etc.) maintain the abrasiveparticles in suspension.

When it is desired to dispense slurry through nozzles 18, pinch bar 92is lifted by actuator 94 to close pinch valves 83' and 85', therebyclosing off flow in outflow line 83 and return line 85. At the sametime, pinch valves 89'-91' are opened. Pressurized air therefore flowsthrough air supply line 91 to junction point 88, where it forces theabrasive slurry in dispense section 84 on either side of junction point88 through feed lines 89 and 90 and nozzles 18. The timing of actuator94 is set such that air continues to flow until the slurry is entirelyexpelled from feed lines 89 and 90 and nozzles 18. Pinch bar 92 thenreturns to its lower position, closing off lines 89, 90 and 91. The airflow in line 91 is terminated, and no further flow of air or slurry mayoccur in feed lines 89 and 90. The slurry again circulates through lines83 and 85 and dispense section 84. Pump 82 normally need not be shut offwhen pinch valves 83' and 85' are closed because these valves are closedonly during the dispense cycle, which lasts only a few seconds.

Since no slurry remains in lines 89 and 90, the volume of slurrydispensed during each texturing cycle is determined by the volume indispense section 84 between central junction point 88 and junctionpoints 86 and 87, respectively. The volume of slurry dispensed cantherefore easily be adjusted by varying the lengths of these segments.Otherwise, a consistent volume of slurry will be delivered during eachtexturing cycle. Moreover, since feed lines 89 and 90 and nozzles 18 areemptied after each cycle, all of the slurry in the system iscontinuously recirculated and no settling can occur. The relativelyshort time periods during which the slurry is being expelled fromdispense section 84 are not long enough to allow settling in theremainder of the recirculation loop. Therefore, the slurry remains ahomogeneous, uniform mixture and no large, agglomerated abrasiveparticles can form to create problems in the texturing process.

Texturing unit 10 and slurry delivery system 80 may be used together ina disk texturing system which overcomes all of the problems mentionedabove. Numerous alternative embodiments of the structures disclosedherein will be apparent to those skilled in the art. The abovedisclosures are therefore intended to be illustrative only, and notlimiting, and the broad principles of this invention, as defined in theclaims, are intended to include all such embodiments.

We claim:
 1. A system for delivering a preselected amount of a slurry,said system comprising:a slurry reservoir; a pump; a slurryrecirculation loop comprising outflow and return lines and a dispensesection, said dispense section being bounded on either end by a dispenseaperture; means for alternately allowing or preventing flow in both ofsaid outflow and return lines; and means for admitting a gas to saiddispense section so as to expel said slurry through said dispenseapertures while flow is prevented in said outflow and return sections.2. The system of claim 1 wherein said slurry contains abrasiveparticles.
 3. The system of claim 2 wherein each of said dispenseapertures is connected to a slurry feed line, each of said slurry feedlines terminating in a slurry applicator.
 4. The system of claim 3wherein said gas is air and comprising an air supply line connected tosaid dispense section.
 5. The system of claim 4 comprising a means foropening and closing said slurry feed lines and said air supply linesimultaneously.
 6. The system of claim 5 wherein said means for openingand closing comprises pinch valves in each of said slurry feed lines andsaid air supply line.
 7. The system of claim 6 comprising a pinch barfor closing each of said pinch valves simultaneously.
 8. The system ofclaim 7 comprising a pinch valve in each of said outflow and returnlines.
 9. The system of claim 8 wherein said pinch bar operates so as toclose said pinch valves in said slurry feed lines and said air supplyline while opening said pinch valves in said outflow and return lines,and to close said pinch valves in said outflow and return lines whileopening said pinch valves in said slurry feed lines and said air supplyline.
 10. A method of delivering a preselected amount of a slurry, saidmethod comprising:providing a slurry recirculation loop and a pair ofdispense lines extending from first and second points along saidrecirculation loop, said first and second points defining a dispensesection of said loop; recirculating a slurry in said loop; causing saidrecirculation to terminate in said dispense section of said loop; andinjecting a gas into said dispense section so as to expel the slurry insaid dispense section through said dispense lines.